1. Field of the Invention
The present invention relates to a method of preparing styrene polymer, and more particularly, to a method of preparing styrene polymer having high syndiotacticity.
2. Description of the Related Art
In general, styrene polymers are prepared through a polymerization reaction using metallocene catalysts. Metallocene catalysts are activated by alkylaluminum compounds, for example, methylaluminoxane, and contain Group 4 transition metal bonded to a cycloalkandienyl group or a derivative thereof.
Preparation of styrene polymer having syndiotacticity accompanies gelation, and thus the gel should be prevented from growing into a whole lump of gel, which is well known in the art. For this purpose, U.S. Pat. No. 5,037,907 discloses a vertical tank type reactor including a mixer that scratches an inner wall of the reactor, and a method of preparing styrene polymer using the reactor.
According to U.S. Pat. No. 5,037,907, by using the reactor as a polymerization medium, a great amount of polymerization particles are dispersed in a liquid state of monomers to induce polymerization at the particle surface, and thus in theory, quick formation of polymer lumps can be prevented. In fact, however, formation of such large particles or lumps cannot be prevented in a low conversion region. In particular, when polymers are insufficiently mixed, polymers are more likely to be lumped.
U.S. Pat. No. 5,254,647 discloses a wiped surface reactor that is used to prepare styrene polymers. The wiped surface reactor mixes monomers using two pairs of conventional screws to prevent coagulation at low conversion range, where particle growth begins rapidly. Then, the agitated styrene polymers are transferred to a powder bed type reactor. As a result, a high conversion ratio can be obtained.
Although the powder bed-type reactor shows low mixing efficiency, a relatively uniform product can be obtained. In addition, by using the wiped surface reactor, rapid polymerization is prevented and thus it is possible to control the polymerization with ease. However, the rotation of the reactor is limited according to the structure of paddles installed in the cylinder of the reactor. As a result, the reactants loaded into the cylinder have insufficient residence time for reaction. In addition, the use of the screw structure leads to a reduced treatment capacity and high manufacturing costs.
As other references relating to manufacturing of styrene polymers, there are U.S. Pat. No. 5,484,862 disclosing an improved liquid phase powder bed type reactor and U.S. Pat. No. 6,242,542 disclosing back-mixed reactors that are reactors in series or parallel reactors.
Conventional methods of preparing styrene polymers having syndiotacticity, however, accompanies formation of gel, and large particles having an average particle diameter of 2 mm or more are attached to an inner wall or agitator of a reactor in a lump form. As a result, the transfer of a reactant is difficult and in the subsequent reaction, mixing is difficult.
Accordingly, preparation of styrene polymers according to these patents described above requires expensive apparatuses and thus the prepared styrene polymers has low polymerization productivity. In addition, there is a danger of deterioration of the product property caused by the incapability of preventing the gelation.